This invention relates to radio antennas, and particularly to antennas whose size is reduced relative to the nominal operating frequency, or wavelength, of the antenna with minimal resistive power loss.
In radio communications it is often desirable to minimize the size of a radio antenna. This may be the case, for example, to fit the antenna into a limited space, to reduce the material cost of the antenna or its support system, or simply to reduce obtrusiveness of the antenna. At the same time, there is a need to achieve maximum power transfer between the radio to which the antenna is connected and free space. Generally, these are conflicting objectives, because maximum power transfer can only be achieved with a perfect, lossless impedance match, which is defeated by deviation from the optimal size of the antenna for a given operating frequency.
It is known that the electrical length of an antenna can be increased by providing a coil inductor in series with the input to the radiating elements. It is also known that, to match the impedance of a combination of radiating elements and series inductors to a standard transmission line having a characteristic impedance of, for example, 50 ohms, a coil inductor may be placed across the input to the antenna. However, while these techniques permit a dipole antenna to be shorter than one-half wavelength at the nominal operating frequency of the antenna, and a monopole antenna to be shorter than one-quarter wavelength at the nominal operating frequency, they also introduce resistive losses and thereby reduce the power radiated by the antenna or, conversely, received by the radio. This is because the coils have capacitive coupling between loops that require an actual coil to be longer than would be required for an ideal inductor, and the wires of the coils must, as a practical matter, have a much lower diameter than the radiating elements, both of which increase the effective resistance that a radio frequency signal encounters. Moreover, coil inductors will radiate and, due to their geometry, vary the radiation pattern of the antenna from the ideal pattern.
Accordingly, it would be desirable to be able to introduce inductance to shorten the required length of an antenna, and to match the input impedance of the antenna to a transmission line, without introducing unnecessary resistance and without degrading the antenna""s pattern.
The present invention satisfies the afore-mentioned desire by providing a low loss, compact radio antenna and antenna loading method. For a monopole antenna a tubular, conductive radiating element is provided whose length is less than one-quarter of the wavelength of the nominal operating frequency of the antenna. A tubular conductive, loading element is disposed within the radiating element and substantially coaxial therewith, the loading element having a first end for connection to a radio and being electrically connected to the interior surface of the radiating element at a position spaced outwardly from the first end so as to provide inductance in series with the radiating element. An elongate conductive shunt element is disposed within the loading element for electrically connecting the interior surface of the loading element from a point therein spaced outwardly from the first end to a mirror image thereof, so as to provide shunt inductance that matches the impedance of the antenna to the impedance of a transmission line connected thereto at the nominal frequency. An electromagnetic mirror is provided in the form of a ground plane, or in the form of a second combination of radiating element and loading element so as to provide a dipole antenna.
In either case, the loading element may be connected to its respective radiating element at the ends thereof, or at a point interior therefrom, to tune the antenna to a different frequency. The antenna may be tuned selectively by using switches or variable positioning devices to change the connections between the loading elements and the radiating elements, or by introducing a conductive or dielectric material between the loading and radiating elements. Capacitive hats may be provided at the outer ends of the antenna to provide increased current flow at the outer ends of the radiators in order to raise the antenna""s radiation resistance and thereby lower its Q.
Accordingly, it is a principal object of the present invention to provide a novel and improved high-frequency radio antenna whose size is reduced for its nominal operating frequency but that provides relatively low power loss.
It is another object of the invention to provide a low-loss, compact monople antenna whose length is substantially less than one-quarter wavelength at the nominal operating frequency.
It is a further object of the invention to provide a low-loss, compact dipole antenna whose length is substantially less than one-half wavelength at the nominal operating frequency.
It is yet another object of the present invention to provide a radio antenna whose length is reduced by providing an elongate, tubular loading inductor in series with a radiating element and an elongate shunt matching inductor across the antenna input wherein the inductors introduce minimal resistive power loss.
It is yet a further object of the present invention to provide a radio antenna whose length is reduced by providing an elongate, tubular loading inductor in series with a radiating element and an elongate shunt matching inductor across the antenna input wherein the loading inductors are shielded from radiation.
The foregoing and other objects, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.